Prepare 3.9.0

[smorimoto]

Summary

Bump version to 3.9.0 for the next release.

Changes since v3.8.0

Features

• Support stable opam 2.6 releases (Support opam 2.6 and default to the HTTP repository #1132)

Improvements

• Use https://opam.ocaml.org as the default repository to enable opam 2.6's archive storage optimisation for HTTP repositories (Support opam 2.6 and default to the HTTP repository #1132)

Internal

• Update dependencies and rebuild action bundles
• Bump Vite Plus from 0.3.0 to 0.3.2 and Knip from 6.33.0 to 6.36.0
• Bump jdx/mise-action from 4.2.5 to 4.3.0 (Bump jdx/mise-action from 4.2.5 to 4.3.0 #1130)

Testing

• yarn install --immutable (passed with a peer dependency warning)
• yarn lint
• yarn knip
• yarn typecheck
• yarn build
• Confirmed rebuilding leaves no additional changes